1. Field of Invention
This invention relates to optics. Specifically, the present invention relates to optical rotators for rotating the polarization state of polarized electromagnetic energy.
2. Description of the Related Art
Polarization state rotators rotate the polarization state of an input beam of electromagnetic energy by a predetermined angle. Rotators are employed in a variety of demanding applications including laser rods, electron linear accelerators, and three-dimensional stereovision video applications. Such applications often require cost-effective space-efficient rotators that can efficiently rotate the polarization state of electromagnetic energy of a particular frequency by a predetermined angle.
Half waveplates are often used to rotate a specific linear polarization state by a specified angle. Unfortunately, rotators employing half waveplates are often ineffective at rotating electromagnetic energy with an arbitrary polarization state by a specified angle.
Quartz optically active rotators are typically employed to rotate the polarization state of an arbitrarily polarized light beam. The quartz rotators are constructed from a slab of high-quality optically active quartz. The thickness of the slab determines the angle by which the quartz rotator rotates the polarization state of input electromagnetic energy. Large slabs of the high-quality optically active quartz are often required to achieve a particular rotation angle. For example, a conventional 90-degree quartz rotator may require a slab of quartz more than 1 centimeter long. This large size requirement increases costs and limits the applicability of the rotators to applications with ample space. In addition, due to frequency limitations of quartz, quartz rotators are often only effective over a relatively narrow range of input frequencies. Consequently, such rotators are inapplicable to systems requiring polarization state rotation of electromagnetic energy outside of their frequency range. Furthermore, the single slab design of the quartz rotator provides limited design features. Consequently, incorporation of quartz rotators into systems such as variable phase rotators and double-interferometric polarizers is often inefficient and expensive.
Other inventions by this Applicant also assigned to Raytheon Company include: xe2x80x9cReeder Rotatorxe2x80x9d (Ser. No. 09/483,254, now U.S. Pat. No. 6,268,962; Attorney Docket No. PD-R98093); xe2x80x9cReeder Rodxe2x80x9d (Ser. No. 09/082,230, now U.S. Pat. No. 6,219,455; Attorney Docket No. PD-R98094); and xe2x80x9cReeder Compensatorxe2x80x9d (Ser. No. 09/482,376, now U.S. Pat. No. 6,317,450; Attorney Docket No. PD-R98098). These applications are incorporated herein by reference.
In some applications, Faraday rotators are employed instead of the conventional optically active quartz rotators. Unfortunately, Faraday rotators are non-reciprocal rotators, such that light reflecting off a mirror and making a second pass through the rotator is rotated in the same direction as the first pass. Faraday rotators are often inapplicable to systems that require reciprocal rotators.
Hence, a need exists in the art for a cost-effective space-efficient reciprocal polarization state rotator that can rotate the polarization state of electromagnetic energy over a wide range of frequencies. There exists a further need for a rotator having flexible design features for accommodating applications such as variable phase rotators and double-interferometric polarizers.
The need in the art is addressed by the system for rotating a polarization state of a beam of electromagnetic energy of the present invention. In the illustrative embodiment, the inventive system includes a first mechanism for receiving the beam of electromagnetic energy, which is characterized by a first polarization state oriented at a first angle. A second mechanism orients the first polarization state, via one or more waveplates, at a second angle that is different from the first angle.
In a specific embodiment, the one or more waveplates include a first quarter waveplate having a principal axis angled at 45xc2x0 from horizontal relative to a given reference frame. A second quarter waveplate is angled at xe2x88x9245xc2x0. A phase mechanism is positioned between the first quarter waveplate and the second quarter waveplate. The phase mechanism introduces a desired phase shift to the polarization state of the beam of electromagnetic energy coming from the first quarter wave plate. The desired phase shift is twice the difference between the first angle and the second angle, which corresponds to a desired angle of rotation of the first polarization state.
In an illustrative embodiment, the phase mechanism includes a fixed-phase waveplate. In another illustrative embodiment, the phase mechanism includes a variable phase device such as a moving wedge waveplate or an electro-optic crystal. In yet another embodiment, the phase mechanism includes an interferometric arrangement that facilitates the production of radially, tangentially, or combined radially and tangentially polarized light.
The novel design of the present invention is facilitated by the use of waveplates to rotate the polarization state of an input beam of electromagnetic energy by a desired angle. This results in a more compact optical device that can accommodate a wider range of frequencies, is more cost effective, and is generally more easily adapted to meet the needs of current demanding applications than its monolithic quartz rotator counterpart.